The present invention relates to a multi-stylus head comprising fine multi-stylus electrodes arranged in a row of a plurality of rows, which is employed for forming electrostatic latent images in an image recording apparatus.
Referring to FIG. 1, there is perspectively shown a conventional multi-stylus head of the above-mentioned type which is described in U.S. Pat. No. 3,653,065. In the figure, a head 1 comprises stylus electrodes 2 arranged in a row with 8 styli per mm. The alignment of the styli starts from the central portion between two segmented electrodes 3.sub.11 and 3.sub.21. The segmented electrodes 3.sub.11 .about.3.sub.1(2n+1) and the segmented electrodes 3.sub.21 .about.3.sub.2(2n+1) are aligned on both sides of the aligned styli, with their centers 6 mm apart. The segmented electrode pairs (3.sub.11 and 3.sub.21) and (3.sub.2(2n+1)), which are disposed on both sides of the styli, are connected to each other. There are disposed 2048 styli in total, which are grouped with each group consisting of 48 styli. Therefore, one group of styli is positioned between the respective centers of the two adjacent segmented electrodes. The groups of the styli can be classified into odd number groups SG.sub.1, SG.sub.3, . . . , SG.sub.2n- 1, and even number groups SG.sub.2, SG.sub.4, . . . , SG.sub.2n. The styli at the corresponding positions in the odd number groups are connected to each other, while the styli at the corresponding positions in the even number groups are connected to each other. Therefore, when a voltage is applied to one common connection wire, the same voltage is applied to all the styli connected to that connection wire. The value of the voltage applied to the styli is such that electrostatic images are not formed by the voltage. When a voltage with an opposite polarity to that of the above-mentioned voltage is applied to the segmented electrodes, an electrostatic latent image is formed. Thus, latent image formation is performed successively in each group of the styli from one end of the stylus electrode 2 to the other end thereof. A dielectric layer of an electrostatic recording paper is brought into pressure contact with the recording end surface of the head 1.
The timing of application of the voltage to each segmented electrode and the timing of application of the voltage to each group of the styli in accordance with the image signals are set as shown in FIG. 2. When developing the electrostatic latent image thus formed, it may occur that the image density becomes uneven in vertically-striped patterns as shown in FIG. 3. The vertically-striped patterns are considered to be formed for the following reason: When recording is started from the stylus group SG.sub.1, voltages are applied to the stylus group SG.sub.1 and the segmented electrodes 3.sub.11, 3.sub.12, 3.sub.21 and 3.sub.22. An equivalent circuit in that case is shown in FIG. 4. When the voltage V.sub.st (multi-stylus application voltage) and V.sub.se (segmented electrode application voltage) are applied, a charge current flows as shown in FIG. 5. However, the electrostatic capacity of a capacitor C.sub.g in the stylus gap between the stylus 2.sub.i and the dielectric layer DL is extremely small compared with the electostatic capacity of a capacitor C.sub.se1 between the segmented electrodes 3.sub.11 and 3.sub.12, and the conductive layer CL, and compared with the electrostatic capacity of a capacitor C.sub.st of the dielectric layer DL below the stylus gap. Therefore, the voltage V.sub.st +V.sub.se is mostly applied to the stylus gap, so that dielectric breakdown and discharging take place in the stylus gap. At that moment, since the resistivity of the conductive layer CL, that is, the value of a resistor R.sub.po, is far smaller than those of the capacitors R.sub.p1 and R.sub.p2, most of the discharge current is the charged current of the electrostatic capacities C.sub.se1 and C.sub.se2. When the voltage has been applied, the stylus 2.sub.i and the segmented electrodes 3.sub.11, 3.sub.21, 3.sub.12 and 3.sub.22 are grounded. As a result, the charges of the capacitors C.sub.se1 and C.sub.se2 are discharged through the resistor R.sub.po as shown in FIG. 6. However, the charges of the capacitor C.sub.st, that is, the charges of the dielectric layer DL, are not discharged, since the insulating properties of the stylus gap have been restored, whereby a latent electrostatic image LI is formed. Generally, the grounded position of the conductive layer CL is located away from the head 1, and, furthermore, the resistivity of the conductive layer CL is so high that the discharging of the capacitor C.sub.se2 through the resistor R.sub.po has not been completed when recording is performed by applying a voltage to the styli of the second stylus group SG.sub.2. Therefore, when the voltage is applied to the styli of the second stylus group SG.sub.2 and to the segmented electrodes 3.sub.12, 3.sub.22, 3.sub.13 and 3.sub.23, the value of the charge current of the multi-stylus on the side of the segmented electrodes 3.sub.12 and 3.sub.22 is decreased due to the effect of the residual charges of C.sub.se2. Accordingly, the charges for forming the latent electrostatic image LI are reduced and the image density of the recorded image is also reduced. As a result, the recording image density becomes less on the recording start side of the multi-stylus in each stylus group following the second stylus group SG.sub.2, so that the vertically-striped patterns appear as shown in FIG. 3.
Therefore, in order to prevent the occurrence of such vertically-striped patterns, a recording method of energizing the stylus groups alternately, for instance, energizing SG.sub.1, SG.sub.3, SG.sub.5, . . . ; and SG.sub.2, SG.sub.4, SG.sub.6, . . . , is proposed in Japanese Laid-open Patent Application No. 136832/1978. According to this method, recording is performed by alternative use of the stylus groups, SG.sub.1, SG.sub.3, SG.sub.5, . . . , and therefore when recording is performed by use of their adjacent stylus groups, SG.sub.2, SG.sub.4, SG.sub.6, . . . , the capacitors C.sub.se1 and C.sub.se2 have been completely discharged, so that the image density does not become uneven. However, generally, the recording sheet is continuously fed during the recording operation at a predetermined speed. Therefore, when recording is performed by use of the stylus groups SG.sub.1 .about.SG.sub.2n-1 and then recording is performed by use of the stylus groups SG.sub.2 .about.SG.sub.2n, there may occur some shift or steps in position between the recording by the odd number stylus groups, SG.sub.1 .about.SG.sub.2n-1, and the recording by the even number stylus groups, SG.sub.2 .about.SG.sub.2n, which may degrade the image density.